This invention relates to electro-optic displays of the type operative either in transmissive or reflective modes, and more particularly to dot matrix displays of the liquid crystal type.
Dot matrix of X-Y matrix displays have been suggested since the inception of liquid crystal displays, as shown in the earliest liquid crystal displays of the dynamic scattering type exemplified by U.S. Pat. No. 3,322,485 issued to R. Williams on May 30, 1967, and in the later field effect twisted nematic liquid crystal displays exemplified by U.S. Pat. No. 3,731,986--Fergason, issued May 8, 1973. It is also known from U.S. Pat. No. 3,540,796--Goldmacher et al that matrix-type liquid crystal displays can be operated in various viewing modes such as reflective, absorbative, or transmissive. It is further known that the image being displayed on a liquid crystal display may be reversed with respect to the background, either by changing the orientation of one of the polarizers so that the axes are either crossed or uncrossed as disclosed in the aforementioned Fergason patent, or by reversing the sign of the signal voltage applied to a matrix-addressed liquid crystal display, as shown in U.S. Pat. No. 3,936,815--Kogure et al issued Feb. 3, 1976. An improved liquid crystal display, operable for either day or night viewing by means of rotating the polarizers and providing illumination means on the display is disclosed in U.S. Pat. No. 3,950,078 issued Apr. 13, 1976 to Zatsky and assigned to the present assignee.
Finally, the prior art shows means of improving contrast of displays by measuring ambient illumination and adjusting the display brightness as shown in U.S. Pat. No. 4,006,583--Vuilleumier issued Feb. 8, 1977.
In most liquid crystal displays, the display is operated with dark image segments on a light field. This appears to yield greater perceived contrast than obtained with light (transparent) segments on a dark field when the display is operated in a reflective mode in ambient light. On the other hand, in a transmissive mode with an illuminating device behind the display, light (transparent) segments on a dark field produce greater perceived contrast. Since electro-optic displays operate under varying ambient light conditions, it would be desirable to have a display which operates under the optimum image contrast ratio automatically.
Accordingly, one object of the present invention is to provide an improved display having better perceived image contrast ratio under all types of ambient light conditions.
Another object of the invention is to provide an improved dot matrix liquid crystal display for either day time or night time viewing of the displayed image with optimum visibility.